1. Field of the Invention
This invention relates to tank type voltage controlled oscillator circuits and, more specifically, to such circuits capable of starting up and free running from a known initial state and being stopped within two periods of the oscillator response frequency by invoking a stop signal.
2. Brief Description of the Prior Art
Prior attempts to solve the problem of rapidly stopping the oscillation in a resonating coil/capacitor tank were generally avoided in such oscillator circuit designs. Prior art oscillator circuits generally employed a voltage controlled multivibrator which did not use the coil and therefore did not have the large amount of electrical inertia that is present in a tank type configuration. However, the use of such tank type oscillator circuits and their inertia are beneficial when voltage controlled oscillator noise immunity is considered.
One prior art circuit, which is set forth in FIG. 1, uses a different zero phase start up on an LC tank. When this circuit clamps or stops oscillation of the oscillator, the voltage across the resistor Rc and inductor L series combination is:
Vbe67+Vbe68-Vbe66-Vdiode or approximately VBE-Vschottky diode which is approximately 300 mV.
where Vbe67 is the voltage of the base with respect to the emitter of transistor Q67 and Vbe68 and Vbe66 are the same with respect to transistors Q68 and Q66 respectively and Vsdiode=Vschottky diode which is the forward biased voltage drop of diode D104 (anode with respect to cathode)
Therefore, the clamp current is ##EQU1##
The value of Iclamp must match the peak ac coil current, the reason being that any imbalance will affect release time of the clamp by affecting the duty cycle of the oscillator's first cycle.
The above noted circuit performs its intended function, however it also requires a clamp of the oscillator output because this clamp configuration usually required three to five full oscillator periods to stop the resonant oscillation of the LC tank (and its effect on the output). This presents a problem in many environments and applications.